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Apatite fission-track thermochronology of the Pennsylvania Appalachian Basin
Geomorphology, 2 (1989) 39-51 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
39
Apatite Fission-Track Thermochronology of the Pennsylvania Appalachian Basin MARY K. R O D E N a n d D O N A L D S. M I L L E R Department of Geology, Rensselaer Polytechnic Institute, Troy, N Y 12180-3590 (U.S.A.) (ReceivedDecember 12, 1988, accepted after revision May 1, 1989)
Abstract Roden, M.K. and Miller, D.S., 1989. Apatite fission-track thermochronologyof the Pennsylvania Appalachian Basin. In: T.W. Gardner and W.D. Sevon (Editors), Appalachian Geomorphology.Geomorphology,2:39-51. Thirty-four apatite fission-track apparent ages and twenty-four track length distributions for ash bed samples from the Valleyand Ridge Province and Upper Devonian to Upper Pennsylvanian sedimentary samples from the Allegheny Front and AlleghenyPlateau of Pennsylvania suggest that these regions represent different thermal (uplift) regimes as well as different structural provinces. The Valleyand Ridge Province Tioga and Kalkberg ash bed samples yield apatite fission-track apparent ages and track length distributions that indicate early post-Alleghanian (285-270 Ma) cooling and unroofing that began at ~ 250 Ma. Assuming a geothermal gradient of 25 °C km-1, a burial depth of at least 3.4 km can be estimated for all the Pennsylvania samples. At the Alleghenystructural front and on the western AlleghenyPlateau, the apatite fission-track apparent ages ( < 150 Ma) and track length measurements indicate a Late Jurassic-Early Cretaceous thermal event for these samples possibly resulting from a higher geothermal gradient coinciding with kimberlite intrusion at this time along the Greene-Potter Fault Zone. In northeast Pennsylvania on the AlleghenyPlateau, the Upper Paleozoicsedimentary samples yieldapatite fissiontrack apparent ages < 180 Ma. Narrow track length distributions with long mean lengths (13-14 #m) and small standard deviations (1.3/zm) suggest rapid cooling from temperatures > 110°C during the Middle Jurassic-Early Cretaceous for this part of Pennsylvania. This is consistent with the suggesteduplift history of the Catskill Mountain region in adjacent New York State.
Introduction E s t i m a t i o n of paleoburial depths in the Appalachian Basin have been based on c o n o d o n t alteration indices (Harris, 1979), vitrinite reflectance anisotropy (Levine, 1986), fluid inclusions (Orkan and Voight, 1983), apatite fission-track analysis ( L a ka t os and Miller, 1983; Miller an d Duddy, 1989), s e di m ent ar y compaction (Paxton, 1983), a nd geodynamic lithospheric flexure modelling ( B e a u m o n t et al., 1987). T h e results have indicated a general eastward-thickening (i.e., burial t e m p e r a t u r e
0169-555X/89/$03.50
increasing eastward) depositional basin. J o h n s s o n (1986), in an apatite fission-track and clay mineral diagenesis study of the Middle Devonian Tioga Ash Bed in New York State, found evidence of a t h e r m a l anom al y west of Syracuse t h a t could not be explained solely by burial depth. T h e existence of this t h e r m a l anom al y suggested re-evaluation of the concept of a simple eastward-thickening depositional basin. T hi s paper presents a t h e r m a l history for the central Appalachian Basin of Pennsyl vani a based on apatite fission-track t h e r m o c h r o n o l -
© 1989 Elsevier Science Publishers B.V.
40
MARYK. RODENAND DONALDS. MILLER
ogy of samples of primarily the Middle Devonian Tioga Ash Bed, and Upper Devonian Catskill Formation, as well as a few samples of other Upper Paleozoic sedimentary formations. Samples were taken from the Allegheny Plateau, Allegheny Front, and the Valley and Ridge Province of central and eastern Pennsylvania (Fig. 1 ). This sampling procedure was designed to test the concept of increasing temperatures in the eastern Appalachian Basin due to increasing burial depth and to detect any thermal anomalies that exist across the basin. The fission-track method is based on the formation of damage-zones in uranium-bearing minerals resulting from the spontaneous fission of 23sU over geologic time. Apatite contains appropriate levels of uranium for fission-track analysis. A measured fission-track age is a result of both the time interval of track retention and the amount of annealing that has occurred during that time (Green, 1988). The fissiontrack age will be completely reset, if sufficient heat is applied to totally erase all existing fission-tracks in the apatite. Below a certain critical temperature, known
as the closure temperature (Dodson, 1973, 1979), fission-tracks will begin to accumulate. In isotope geochronology, closure temperature represents the time at which the radiogenic daughter product no longer diffuses out of the system but begins to be retained. In the case of fission-tracks in minerals, this is equivalent to the retention of the damaged regions referred to as tracks. For apatite, the closure temperature is estimated to be 100_+20°C (Wagner, 1968; Naeser and Faul, 1969; Gleadow and Duddy, 1981; Naeser, 1981). Apatite fissiontrack apparent ages cited in this study (Table 1; Fig. 1 ) reflect the time those apatites cooled through the closure temperature of 100°C not their depositional age. Fission-track length measurements on confined, spontaneous tracks in apatite combined with the apatite fission-track apparent ages have been used in thermal history interpretations (Gleadow et al., 1983, 1986a, b). Track length distributions from rapidly cooled volcanic apatites which have not been exposed to temperatures > 50 ° C subsequent to initial cooling have mean track length values between
Falls Creek Wyalusing
152
155
"~ ..
A L L E G H E N Y P L A T EAU
• • F~Sl"" ".Avoc a •• " 14 ~ Pittstowd 127 ~ . • U . • ,• / Dutx~s Lock Haven &. - - - - 121 , ./2" 141 L.^ 97. " " BartSdsvilleY • '-°.~135.-~ V A L L E Y & RIDGE ,, ,' 1 8 4 JStroudsburA L L E G H E N Y FRONT ~.&' Selinsgrove "'" ~1177 = ~ ,'" 218 144-~ urazierville"~," • Kitattiny Mt.) 233 Wardville ,," ~Altoona ,'e Old Port 1 8 7 Swatara Gap..-" /~ JohP.stown4 4 " Mapleton 2 4 5 Mi~,..^. 1 5 2 .--> ..;;-:': -~, Powrys
7&8 ,~'~46 2 2 4 = 2 2 5 2 ~ 5 GR I~AT V A L L E Y L ~ Balc~,'HiU ~ Newton-Hamilton -" . .... ~_-~......~ .~ . .,,' Entr/~iklen892~rbisoni.&..~;,.TR[.ASSiC G E T T Y S B U R G ' ~ A S I N Dickey's Mt,/
203,11 -.
233
=
/'
/
,:
~
~,
." ' A i
'
'" /
PIEDMONT -
J
-
BLUE RIDGE 0
50
100 km
Fig. 1. Sample location names for Tioga ( • ), Kalkberg (II), and Upper Paleozoic sedimentary samples ( • ), apatite fissiontrack apparent ages (Ma), and major tectonic provinces in Pennsylvania.
APATITEFISSION-TRACKTHERMOCHRONOLOGYOFTHE PENNSYLVANIAAPPALACHIANBASIN
41
TABLE1 Apatite fission-track apparent ages, mean track lengths, C1/C1 + F ratios, and U concentrations for the Pennsylvania Appalachian samples Sample number (elev.)
Formation
Sample location
Apparent apatite fission-track age in Ma (no. grains)
Mean track length in/~m (no. tracks)
C1/F+C1
U (ppm)
152_ 8 (20) 245 ___13 (15) 225 _+13 (20) 205 _+12 (19) 233 _+15 (19) 224 _+16 (16) 233 + 13 (19) 218 _+14 (20) 187 + 12 (15) 215 _+13 (12) 177 -+ 10 (18) 203 -+22 (15) 246 _+26* (18) 189 +__22* (19)
13.23_+ 2.0 (98) 14.05 _+1.7 (101) 13.17 ___1.9 (98) 13.48 +__1.6 (101) 13.47 +_1.4 (103) 13.52 -+ 1.6 (100) 13.54 _+1.8 (100) 13.74 + 1.5 (100) 13.78 _+1.5 (101) 13.62 _+1.6 (84) 13.22 _+1.6 (60) 13.21 _+1.6 (130) 13.34 _+1.8 (114) 12.46 _+1.7 (26)
0.19
34
0.19
33
14.04_+ 1.2 (99)
Valley and Ridge Province samples 84-92 (171 m) 8500-1 (135 m) 8500-2 (208 m) 8500-3 (104 m) 8500-4 (190 m) 8500-5 (239 m) 8502-2 (294 m) 8502-3 (141 m) 8602-3 (184 m) 8602-4 (214 m) 8602-5 (141 m) 8602-9 (294 m) 8602-10 (306 m) 8602-52 (245 m)
Tioga
Swatara Gap
Tioga
Old Port
Tioga
Newton-Hamilton
Tioga
Midway
Tioga
Dickey's Mt.
Tioga
Mapleton
Tioga
Grazierville
Tioga
Selingsgrove
Tioga
Wardville
Tioga
Orbisonia
Tioga
Stroudsburg
Kalkberg
Hyndman
Kalkberg
Bald Hill
Catskill
Entriken
Northeastern Allegheny Plateau samples 84-79 (404 m) 84-80 (263 m) 84-82 (367 m) 84-83 (251 m) 84-84 (184 m) 84-87 (208 m) 84-89 (588 m)
Shawangunk
Kitattiny Mt.
Catskill
Bartonsville
Catskill
Avoca
Pottsville
Pittstown
Catskill
Falls
Pocono
Wyalusing
144_+ 8 (21) 184 _+17 (14) 127 _+10 (12) 121_+ 7 (20) 141 _+11 (10) 152 _+17
Pottsville
Falls Creek
155 _+12
13.25 _+1.4 (100) 13.63_+1.1 (94) 13.03 ___1.5 (75) 13.60 ___1.2 (103) 13.60 _+1.2 (100)
28 24 28 29 0.16
33 39 44 37
0.19
47
0.21
10
0.20
18
0.06
27
42
MARYK. RODENANDDONALDS. MILLER
TABLE 1 (continued) Sample number (elev.)
Formation
Sample location
Apparent apatite fission-track age in Ma (no. grains)
Mean track length in/~m (no. tracks)
CI/F+CI
U (ppm)
12.25 ± 1.7 (103) 12.32 ± 1.7 (23) 12.38 ± 1.7 (73)
0.05
21
0.01
26
0.02
18
0.01
13
0.02
31
0.003
25
Western Allegheny Front and Allegheny Plateau samples 8702-2 (203 m) 8702-5 (456 m) 8702-7 (612 m) 8702-8 (685 m) 8702-11 (447 m) 8702-16 (220 m)
Catskill
Lockhaven
Catskill
1-80
Glenshaw
Dubois
Burgoon
Altoona
Catskill
Johnstown
Catskill
Powrys
97 ± 10 (10) 135 ± 20* (9) 141 _+12" (20) 44 _+ 7 (10) 78 +_ 8 (15) 140 ± 11 * (20)
12.81 _+1.5 (57) 12.84 ± 1.7 (100)
*The grain ages for these samples exceeded the limits defined by a chi-square test, thus mean ages are given rather than pooled ages.
14 and 15 ]~m and standard deviations that range from 0.8-1.3/lm (Gleadow et al., 1983, 1986a). Confined track length distributions for apatites from plutonic rocks have mean track lengths in the range of 12-14/~m and standard deviations between 1.0 and 2.0/tm (Gleadow et al., 1986a). In studies of apatites from borehole samples of the Otway Formation in the Otway Basin (Gleadow and Duddy, 1981; Gleadow et al., 1983), the mean confined track length shows a progressive and continuous reduction with increasing down-hole temperature from 15/~m for surface and shallow samples to zero at temperatures of 120°C and greater. Gleadow et al. ( 1986b ) show that small degrees of natural annealing produce narrow track length distributions which become progressively shorter and broader as annealing proceeds. These empirically-determined and naturally-observed characteristics of confined track length distributions in apatite described in the previous two paragraphs will be compared to those track length distributions measured in this study of Pennsylvania rocks. Composition (F and C1 concentration) has been shown to affect the individual fission-track
grain ages of apatites (Green et al., 1986). In their study of apatites from the Otway Group sandstone, Green et al. (1986) found the C1rich grains which had been subjected to a thermal history identical to that of the F-rich grains, had fission-track ages closer to the depositional age of the formation. F and C1 concentrations have been determined for representative samples from this study to check the range of C1 variation. Methods
Apatites and zircons for fission-track analysis were isolated after disaggregation from I to 2 kg samples by standard heavy liquid and magnetic separation techniques. Sample preparation procedures for age and trace length measurements were those of Gleadow (1984). Suitable apatite were recovered from 34 out of 44 samples. Twenty-four samples had statistically significant confined track length measurements. Ten-twenty mg aliquots of apatite were mounted in epoxy on glass slides and polished to expose internal grain surfaces. Spontaneous fission-tracks in the apatites were revealed by etching for 20 s in 5M HNO3 at 21 ° C.
APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE PENNSYLVANIA APPALACHIAN BASIN
The external detector method used in this study requires the mount to be covered by a thin sheet of low uranium muscovite secured by tape. The muscovite serves as a detector for the neutron-induced 235U fission fragments. Samples were irradiated with thermal neutrons at either the U.S. Geological Survey TRIGA reactor in Denver or the Oregon State University TRIGA reactor in Corvallis with two pieces of either NBS standard glass SRM 612 or the Coming Glass standard CN1 included at the ends of the irradiation package. A thermal neutron fluence on the order of 8 × 1015 n cm -2 was obtained for apatite samples. Mica detectors were etched in 40% HF for 15 min. Ages were calculated using a weighted mean zeta calibration factor (Fleischer et al., 1975; Hurford and Green, 1983) for standard apatites from the Fish Canyon Tuff, Durango (Mexico), and the Mount Dromedary quartz monzonite (Miller et al., 1985). Track densities (tracks cm-2) of both spontaneous and induced tracks were measured on a Leitz Ortholux microscope at 1600 X (160 × dry objective, 10× oculars). A chi-square statistic was calculated to discriminate between grains that belong to a single age population (p > 0.05) and those that represent a mixed age population (p < 0.05; indicated by an asterisk in Table 1 ). Length measurements on confined horizontal tracks were made at 1563 × (100 X dry objective, 1.25 × microscope tube, 12.5 × oculars) using a Leitz Ortholux microscope with a drawing tube and a Houston Instruments 1011 digitizing pad interfaced with an IBM PC XT computer. Results
Apatite fission-track apparent ages The Tioga Ash Bed and Kalkberg Formation ash bed samples from west-central Pennsylvania display a range of apatite fission-track apparent ages of 215 _+13-246_+ 26 Ma (Table 1; Fig. 1 ). Near the Southern Anthracite Field in
43
east-central Pennsylvania, the Tioga Ash Bed samples yield a younger range of apatite fission-track apparent ages of 152+8-218_+14 Ma. These ages are consistent with the 177 _+10 Ma apatite fission-track apparent age from the easternmost sample at Stroudsburg. A concordant U - P b date of 390.0 _+0.5 Ma (Roden, 1989) for a monazite from bed B (Way et al., 1986) of a Pennsylvania Tioga sample tightly constrains the stratigraphic age for the formation. All apatite fission-track apparent ages determined in this study are significantly younger than this depositional age. The sedimentary samples of Upper Devonian Catskill Formation and other Upper Paleozoic sedimentary formations from the Allegheny Front and Allegheny Plateau yield a range of apatite fission-track apparent ages, 44 _+7-141 _+12 Ma, significantly younger than the Tioga and Kalkberg samples located to the east (Table 1; Fig. 1). Sedimentary samples from the Allegheny Plateau of northeastern Pennsylvania show a range of apatite fissiontrack apparent ages of 121_+7-184_+17 Ma (Table 1; Fig. 1).
Track length distributions The Tioga Ash Bed and Kalkberg Formation ash bed samples from the Valley and Ridge Province of Pennsylvania have measured track lengths that range from 13.2 to 14.0/~m with the mean standard deviation of the track length distributions ranging from 1.4 to 2.0/tm (Table 1). The mean track length is 13.5_+0.3 /tm (probable error of the mean; Table 1 ). The frequency distributions are characterized by negative skewness and similar histogram shape (Fig. 2a). The Upper Paleozoic sedimentary samples from the Allegheny Plateau of northeastern Pennsylvania (northeast plateau) yield a mean length of 13.5 + 0.3/an consistent with the mean track lengths for the ash bed samples but have a much smaller standard deviation of 1.3/tin (Table 1; Fig. 2b).
44
MARY K. RODEN AND DONALD S. MILLER
(e)
40 '1-
Tioga-Old Port, Pa. No. Tracks = 101
0
30
Mean = 14.05
Std. Dev. = 1.68
~5
20
0
10"
D"
Compositional variation
0 dl
Track Length (microns)
(b) Pocono-Wyalusing, Pa. 40 tO
No. Tracks = 103
3O
Mean = 13.60 Std. Dev. = 1.21
2O O"
10-
0
These differences, combined with the distinctly younger apatite fission-track apparent ages for the Allegheny Front and Allegheny Plateau samples (Table 1) suggest that they have experienced a different thermal history from that of the Valley and Ridge Province ash bed samples and the sedimentary samples from northeastern Pennsylvania.
t o t n ~ t ~ t ~ ~ t¢~ t ~ t o t ~ LgJ ~ t ~ ~ ~tP a'~ w~ a'~ t n u'~
@
The Tioga Ash Bed and Kalkberg Formation apatites are monocompositional with a range of CI/CI+F composition values from 0.18-0.21 (Table 1 ). The Upper Devonian and Upper Paleozoic samples from the western Allegheny Plateau and Allegheny Front are fluorapatites with a limited range of Cl/C1 + F ratios (0.0030.06; Table 1). Because of the limited range of C1/CI+F content of the apatites studied, the fission-track apparent ages and track length reductions measured will be the result of thermal effects without the bias of compositional variation.
Track Length (microns)
(c) Catskill-Lock Haven, Pa. 40 ]
30
I
No. Tracks = 103
Valley and Ridge Province
Mean = 12.25 Std. Dev. = 1.66
20" c
10LL
0
Discussion
~ m ~ m m m ~ m ~ m ~ m m m ~ m
Track Length (microns)
Fig. 2a. Track length distribution for (a) Tioga Ash Bed sample for Old Port, Pa.; ( b ) Pocono Formation sample from Wyalusing, Pa.; and (c) Catskill Formation sample from Lock Haven, Pa.
In contrast, the Upper Devonian and Upper Paleozoic samples from the Allegheny Front and western Allegheny Plateau yield a shorter mean track length of 12.5 _+0.3/Jm and a large standard deviation of 1.6 /xm (Table 1; Fig. 2c).
In Fig. 3, the correlation between coal rank, estimated burial depth (Beaumont et al., 1987) and apatite fission-track apparent age is shown. Apatite fission-track apparent ages for the Tioga samples decrease slightly from west to east as maximum estimated burial depth increases from 5 km at Grazierville (233 _+13 Ma) to 9 km at Stroudsburg (177+10 Ma). The Tioga samples with the youngest apatite fission-track apparent ages are located in the vicinity of the Southern Anthracite Field where both Beaumont et al. (1987) and Levine ( 1986 ) estimate burial depths to be as great as 9 km. Using a geothermal gradient of 25 ° C k m {Beaumont et al., 1987) and a surface temperature of 15°C, a burial depth of at least 3.4 km can be estimated for all the Pennsylvania samples. This is a minimum estimate of burial depth using the apatite fission-track closure temper-
APATITEFISSION-TRACKTHERMOCHRONOLOGYOF THE PENNSYLVANIAAPPALACHIANBASIN
Bituminous Coal Fields
Anthracite Coal F~elds
HighVolath Medium Volatle Low Volatile 0 50 IL
45
,
~ ~
Anthradte Semi-Anthracite
100 km ,
Fig. 3. Apatite fission-track.apparent ages (Ma) for Tioga ( • ) , Kalkberg ( • ) , and Upper Paleozoic sedimentary samples ( • ) from Pennsylvania, estimated maximum burial depth contours in parentheses (kin) (Beaumont et al., 1987), and coal rank.
ature of 100 ° +_20°C. It is consistent with the models of Beaumont et al. (1987) and Levine (1986), however, their maximum burial depth estimates cannot be confirmed because the temperature at 5-9 km depth is >> 100°C (fission-track retention in apatite). Both paleomagnetic ages of 285-270 Ma (Late Pennsylvanian-Early Permian, Van der Voo, 1979; Kent, 1979; Scotese et al., 1982) and vitrinite anisotropy data (Levine, 1986) limit the overburden emplacement and anthracite formation to a 10-15 Ma period. The apatite fission-track apparent ages for the ash bed samples from the Valley and Ridge Province (187+_11-246+_ 26 Ma), with the exception of the Swatara Gap and Stroudsburg samples (Table 1 ), suggest that uplift and erosion in this region began immediately after Alleghenian folding (285-270 Ma) which is consistent with the estimated short time interval (15 Ma) for maximum burial and deformation. The apatite
fission-track apparent ages are also consistent with the calculated erosion period for the ancestral Appalachians (Slingerland and Furlong, this volume). The apatite fission-track apparent ages of ~ 220 Ma indicate an overburden of approximately 3-4 km (depending on estimated geothermal gradient) existed at that time. Slingerland and Furlong (this volume) suggest that the ancestral Appalachians would have been eroded to their present level by 230 Ma which may be consistent with the fissiontrack data within the errors of the model assumptions. The Tioga samples from Swatara Gap and Stroudsburg with the youngest apatite fissiontrack apparent ages (152 + 8 and 177 + 10 Ma, respectively) are located on the southeastern edge of the Valley and Ridge Province near the Triassic Gettysburg Basin (Fig. 1). Their younger apatite fission-track apparent ages may reflect the effects of higher heat flow, higher pa-
46
leogeothermal gradient, in this region during the Mesozoic as suggested by Kohn et al. (1988a, b). Reset fission-track ages for sphene (mean age= 197 Ma) and zircon {mean age= 182 Ma) for samples of Grenville and Taconic crystalline rocks and the Mesozoic Newark Series from the Piedmont and southeastern Pennsylvania have been reported by Kohn et al. (1988a, b). The apatite fission-track apparent ages for their samples have a mean age of 146 Ma and mean track lengths of 13-14/lm. They attribute these fission-track ages to rapid cooling following a high thermal regime (50°C km -1) during the Late Triassic-Early Jurassic extension along the Atlantic margin. From the zircon and sphene fission-track apparent ages, Kohn et al. (1988a, b) estimate 610 km of uplift in the central Appalachian Piedmont since early Jurassic time. Their apatite fission-track apparent ages and track length distributions indicate a discrete phase of Late Jurassic-Early Cretaceous uplift. The apatite fission-track apparent ages for the Swatara Gap and Stroudsburg samples are consistent with a Middle to Late Jurassic uplift in southeastern Pennsylvania. The measured track length distributions for the Valley and Ridge Province samples have long mean lengths (13.2-14.1 ttm) and large standard deviations (1.3-2.0 ttm; Table 1 ). This combined with a frequency distribution shape that is skewed toward short track lengths (Fig. 2a) suggests a slowly decreasing cooling path from temperatures of ~ 120°C to present surface temperature (15 ° C ).
Western Allegheny Front and Allegheny Plateau Figure 3 shows the apatite fission-track apparent ages for the sedimentary samples from the northwestern to northeastern Allegheny Plateau and the Allegheny Front of Pennsylvania and the predicted amounts of post-A1leghenian erosion (Beaumont et al., 1987).
MARY K. RODENAND DONALDS. MILLER
Maximum estimated burial depths vary from 3 km west of the Allegheny Front at Dubois to 9 km in the east near the Northern Anthracite Field. The apatite fission-track apparent ages for all the Allegheny Front and Allegheny Plateau samples are significantly younger than those from the Valley and Ridge Province samples (Fig. 1; Table 1). This is inconsistent with a model of uniform uplift across an eastward deepening depositional basin because the samples in the western part of the basin (western Allegheny Plateau) should have apatite fission-track apparent ages that are the same as, or older than, those of the northeastern Plateau and Valley and Ridge Province samples ( > 246 + 26 Ma). Instead, all the western Allegheny Plateau samples yield apatite fissiontrack apparent ages < 150 Ma. The mean track lengths (12.5+0.3/lm) for the western samples are also shorter than those of both the northeastern Plateau and the Valley and Ridge Province samples (Table 1). Mean track lengths in the 12 #m range with broad frequency distributions suggest slow-cooling with a relatively long time spent in the "track annealing zone" (70 ° -90 ° C ). A mechanism besides burial depth is needed to create a thermal perturbation in the western Allegheny Front and Allegheny Plateau region. There is recent evidence supporting the model of Oliver (1986) for orogenic dewatering due to thrust sheet emplacement which causes topographic highs and forces connate brines toward the craton. The fluid flow resulting from dewatering of the Appalachian Orogen has been suggested as a cause of a Kiaman remagnetization event in the Permo-Carboniferous (310250 Ma) (Miller and Kent, 1988; Jackson et al., 1988), to form authigenic K-feldspars (322-278 Ma, 4°Ar/39Ar ages) (Hearn and Sutter, 1985; Hearn et al., 1987 ), and to act as an illitization agent on mixed-layer illite-smectite in Middle Ordovician K-bentonites with K / A r ages of illitization between 272-303 Ma (Elliott and Aronson, 1987; Altaner, 1985).
47
APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE PENNSYLVANIA APPALACHIAN BASIN
Fluids with temperatures in the range of 100°-200°C would have reset the apatite fission-track ages in sediments that they contacted. This means that the fission-track t T history from depositional age of the sediments (374-300 Ma; Palmer, 1984) to fluid infiltration age ( ~ 250-300 Ma) would be erased. The apatite fission-track apparent ages for the western Allegheny Front and Allegheny Plateau samples should be < 245 Ma representing the time when the sediments had cooled to < 100 °C to allow fission-track accumulation. However, the Allegheny Front and Allegheny Plateau samples have apatite fission-track apparent ages that range from 44 + 7 to 141 + 12 Ma (Fig. 1; Table 1 ). This indicates that there has been post-Permian thermal activity in this region a n d / o r the sediments remained buried to depths where the temperature was 70 °-90 ° C (track annealing zone) until the Late JurassicEarly Cretaceous ( < 150 Ma). There is evidence for both these occurrences.
The Allegheny Front and Allegheny Plateau samples from western Pennsylvania are located along an extension of the Rome Trough and near the cross-structural lineaments presented by Parrish and Lavin (1982) (Fig. 4). This corresponds to the keel-line trend of Dennison (1978, 1983). Parrish and Lavin (1982) suggested that the coincidence of the Pennsylvania and New York kimberlite intrusions and the down-to-the-east basement faults and cross-structural lineaments of the Rome Trough indicate reactivation of these faults in response to the Jurassic extensional opening of the Atlantic Ocean. Basu et al. (1984) have presented K-At age determinations for the Syracuse and Ithaca, New York kimberlites which yield an age range of 113 + 11-176 + 17 Ma. This range of K-Ar ages coincides with the range of apatite fission-track apparent ages ( 4 4 + 7 - 1 4 1 + 1 2 Ma) determined for the Allegheny Front and Allegheny Plateau samples.
//
Greene--PotterFaultZone /
~ , /
/ Allegheny Plateau . ~ ~
1
4
1
,
/
"-... /
\
0 w
r' "
,,, " ""',
"'
,*,,.
Dixonvile"0.
Kimberf~es
~-s-""
• " " "t4-O- - - -
"x<
Allegheny Front
Paleozoic S e d i m e n t s F ~ T r a c k Ages(Ma.)
•
50
m
100
i
•
km
Fig. 4. Greene-Potter fault zone, cross-structural lineaments: T - M U = Tyrone-Mt. Union, P - W-- Pittsburgh-Washington, H-G = Home-Gallitzen, B - B = BlairsviUe-Broad Top, R = Roen discontinuity, L - A = Lawrenceville-Attica (Parrish and Lavin, 1982), apatite fission-track apparent ages (Ma), and kimberlite locations.
48
MARY K. RODEN AND DONALD S. MILLER
Glenshaw-Dubois, Pa. o~ ""
No. Tracks = 73
O
'.= "~ J~
~5 >,
Mean = 12.38 30
Std. Dev. = 1.68
20'
u c
g
10
tL "0
~
Northeastern Allegheny Plateau
40
o. - . - - - - ~ - . k ~ d#dd4dd~d~d~d4~dKdd Track
Length
(microns)
Fig. 5. Bimodal track length distribution for a Glenshaw Formation sample from Dubois, Pa.
The intrusion of the kimberlites themselves could not generate enough heat on a regional scale to overprint a post-Permian cooling age (245 Ma) in these sediments. However, the widespread occurrence of these kimberlites along this zone suggests the possibility of an increased thermal gradient associated with regional igneous intrusive activity in the Rome Trough during the Jurassic. The presence of an intersecting fault zone and lineaments provides tectonic pathways for fluid migration. If the fluids were in the temperature range of 70 °90°C, they could cause annealing of fissiontracks in the apatites they contacted. Additional evidence for a post-Permian heating event on the western Allegheny Plateau of Pennsylvania can be seen in the track length distribution for a Glenshaw Formation from Dubois (Fig. 5). It has a bimodal track length distribution with a shorter peak at 10.5 ttm. This type of length distribution is characteristic of apatites which have spent a prolonged period in the 80°C temperature range (Green et al., 1989). It was during the time at 80 °C that the 10.5 pm peak in the bimodal distribution formed. The temperature was not hot enough to completely anneal the tracks formed since the initial post-Permian cooling, but it was sufficiently high to shorten the tracks to the 10/lm range. The higher peak at 13.5 pm formed after the temperature dropped below 80 ° C.
More rapid cooling (uplift) is indicated for the Upper Devonian samples from the northeastern Allegheny Plateau in their narrow length distributions having a mean standard deviation of 1.3 /tm and a long mean length (13.5 + 0.3 #m). Apatite fission-track apparent ages ( 121 + 17-184 _+17 Ma) for these samples are consistent with a Middle Jurassic to Early Cretaceous uplift as suggested by Miller and Duddy (1989) based on apatite fission-track apparent ages for Upper Devonian samples from the Catskill Mountains to the northeast in New York State. Cl/Cl + F ratios It is doubtful that the apatite fission-track age difference between the Allegheny Front and Allegheny Plateau apatites and the Valley and Ridge Province apatites is due solely to compositional differences. All apatites analyzed in this study comprise a very narrow compositional range (C1/CI+F=0.003-0.21) compared to the Otway Group apatites analyzed by Green et al. {1986) (C1/Cl+F=0.01-0.51). Because the Pennsylvania apatites lack a wide range of C1 concentrations, they do not show a significant correlation between increasing C1/ C1 + F ratio and increasing apatite fission-track apparent ages (Roden, 1989) as seen in the Otway Group apatites (Green et al., 1986). Conclusions
Apatite fission-track apparent ages and track length distributions for the Pennsylvania sedimentary and ash bed samples from the Allegheny Front and Allegheny Plateau and Valley and Ridge Provinces suggest that these regions are not only different tectonic provinces but also represent different thermal (uplift) regimes. The Valley and Ridge Province ash bed samples yield apatite fission-track apparent ages and track length distributions that indicate early
APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE PENNSYLVANIA APPALACHIAN BASIN
post-Alleghanian (285-270 Ma) cooling and unroofing beginning at ~ 250 Ma. This is in agreement with the calculated erosion times of Slingerland and Furlong (this volume). Burial depths of at least 3.4 km can be calculated based on an estimated geothermal gradient of 25 °C km -1 for all the Pennsylvania samples. This is consistent with vitrinite anisotropy measurements of Levine (1986) and geodynamic models of Beaumont et al. (1987) and Slingerland and Furlong (this volume) which suggest deep burial (6-10 kin) in the anthracite region followed by rapid unroofing over a 15 Ma period. Interpretation of the thermal history from apatite fission-track thermochronology west of the Allegheny structural front is complicated. The samples from western Pennsylvania along the Allegheny Front and Allegheny Plateau have been reset to give apatite fission-track apparent ages consistent with thermal histories beginning at < 150 Ma. It is possible that these samples have experienced two heating events. One was caused by hot fluid infiltration ( 100 o_ 200 °C) during the time interval ( ~ 300-250 M a ) as a result of orogenic dewatering. These hot fluids would have reset the apatite fissiontrack ages for the sediments from depositional age (374-300 M a ) to _<250 Ma. This event can not be detected by fission-track analyses of apatite due to the following thermal event. A second thermal event at < 150 M a is indicated by the apatite fission-trackapparent ages for these samples. Track length distributions with short mean lengths (12.5 _+0.3 ,,~m),large standard deviations (1.6/~m), and evidence of bimodality indicate that the thermal event caused temperatures to remain in the 80°C temperature range for an extended period during the Late Jurassic-Early Cretaceous. It is suggested that the elevated temperatures m a y have been the resultof a higher geothermal gradient during this time possibly related to kimberliteintrusion along the Greene-Potter Fault Zone and cross-structurallineaments. For the samples from northeastern Pennsyl-
49
vania on the Allegheny Plateau, the narrow track length distributions with small standard deviations (1.3/~m) indicate rapid cooling at < 185 Ma (maximum apatite fission-track apparent age). This is consistent with a Middle Jurassic-Early Cretaceous uplift suggested by Miller and Duddy (1989) for the Catskill Mountain region of adjacent New York State. Rapid uplift in northeastern Pennsylvania as indicated here may correlate with that seen by Kohn et al. (1988a, b) in southeastern Pennsylvania. It may reflect, as Kohn et al. (1988a, b) suggest, post-rifting uplift caused by landward flexural bulging in response to sedimentary loading of the continental margin.
Acknowledgements This research was supported by a Graduate Professional Opportunities Fellowship, 19851987, a Shell Foundation Fellowship, 19871988, a Texaco Research Grant, 1988-1989, a Geological Society of American Research Grant and a Sigma Xi Grant-in-Aid, 1986. We thank John Way of Lock Haven University and Robert C. Smith, II of the Pennsylvania Topographic and Geologic Survey for their help in the sample collection of the Tioga and Kalkberg ash bed samples and I.R. Duddy for measuring apatite fission-track ages and track lengths for the northeastern Pennsylvania samples. We thank W. Sevon, T. Gardner and an anonymous reviewer for their helpful comments and critical reviews.
References Altaner, S.P., 1985. Potassium metasomatism and diffusion in Cretaceous K-bentonite from the disturbed belt, northwestern Montana, and in the Middle Devonian Tioga K-bentonite, eastern U.S.A. Ph.D. Thesis, University of Illinois,Urbana, Ill.,193 pp. Basu, A.R., Rubury, E., Mehnert, H. and Tatsumoto, M., 1984. Sm-Nd, K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution. Contrib. Mineral. PetroL, 86:
35-44. Beaumont, C., Quinlan, G.M. and Hamilton, J., 1987. The
50 Alleghanian orogeny and its relationship to the evolution of the Eastern Interior, North America. In: C. Beaumont and A.J. Tankard (Editors), Sedimentary Basins and Basin-forming Mechanisms. Can. Soc. Petrol. Geol. Mem., 12: 425-445. Dennison, J.M., 1978. Origin of keel-line peridotites in the Appalachian basin. Geol. Soc. Am., Abstr. Program., 10: 387-388. Dennison, J.M., 1983. Reply on "Tectonic model for kimberlite emplacement in the Appalachian Plateau of Pennsylvania". Geology, 11: 252-253. Dodson, M.E., 1973. Closure temperature in cooling geochronological and petrological systems. Contrib. Mineral. Petrol., 40: 259-274, Dodson, M.E., 1979. Theory of cooling ages. In: E. Jaeger and J.C. Hunziker (Editors), Lectures in Isotope Geology. Springer, Berlin, 329 pp. Elliott, W.C. and Aronson, J.L., 1987. AUeghenian episode of K-bentonite illitization in the southern Appalachian basin. Geology, 15: 735-739. Fleischer, R.L., Price, P.B. and Walker, R.M., 1975. Nuclear Tracks in Solids. University of California Press, Berkeley, Calif., 605 pp. Gleadow, A.J.W., Duddy, I.R., Green, P.F. and Lovering, J.F., 1986a. Confined fission-track lengths in apatite-a diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol., 94: 405-415. Gleadow, A.J.W., Duddy, I.R., Green, P.F. and Hegarty, K.A., 1986b. Fission-track lengths in the apatite annealing zone and the interpretation of mixed ages. Earth Planet. Sci. Lett., 78: 245-254. Gleadow, A.J.W., 1984. Fission-track dating methods-II: a manual of principles and techniques. Workshop of Fission Track Analysis: Principles and Applications. James Cook University, Townsville, Qld., 35 pp. Gleadow, A.J.W., Duddy, I.R. and Lovering, J.F., 1983. Fission-track analysis: a new tool for the evolution of thermal histories and hydrocarbon potential. Aust. Petrol. Explor. Assoc. J., 23: 93-102. Gleadow, A.J.W. and Duddy, I.R., 1981. A natural longterm annealing experiment for apatite. Nucl. Tracks, 5: 169-174. Green, P.F., 1988. The relationship between track shortening and fission-track age reduction in apatite: combined influences of inherent instability, annealing anisotropy, and length bias and system calibration. Earth Planet. Sci. Lett., 89: 335-352. Green, P.F., Duddy, I.R., Gleadow, A.J.W., Tingate, P.R. and Laslett, G.M., 1986. Thermal annealing of fissiontracks in apatite: 1. A qualitative description. Chem. Geol. (Isot. Geosci. Sect.), 59: 237-253. Green, P.F., Duddy, I.R., Laslett, G.M., Hegarty, K.A., Gleadow, A.J.W. and Lovering, J.F., 1989. Thermal annealing of fission-tracks in apatite: 4. Quantitative modelling techniques and extension to geological time scales. Chem. Geol. (Isot. Geosci. Sect. ), 79: 155-182.
MARYK. RODENANDDONALDS. MILLER tlarris, A.G., 1979. Conodont color alteration, an organomineral metamorphic index and its application to Appalachian Basin geology. In: P.A. Scholle and P.R. Schluger (Editors), Aspects of Diagenesis. Soc. Econ. Paleontol. Mineral. Spec. Publ., 26: 3-16. Hearn, P.P. and Sutter, J.F., 1985. Authigenic potassium feldspar in Cambrian carbonates: Evidence of Alleghenian brine migration. Science, 228: 1529-1531. Hearn, P.P., Sutter, J.F. and Belkini, H.E., 1987. Evidence for Late-Paleozoic brine migration in Cambrian carbonate rocks of the central and southern Appalachians: Implications for Mississippi Valley type sulfide mineralization. Geochim. Cosmochim. Acta, 51: 1323-1334. Hurford, A.J. and Green, P.F., 1983. The zeta age calibration of fission-track dating. Chem. Geol. (Isotope Geoscience Section), 1: 285-317. Jackson, M., McCabe, C., Ballard, M.M. and Van der Voo, R., 1988. Magnetite authigenesis and diagenetic paleotemperatures across the northern Appalachian basin. Geology, 16: 592-595. Johnsson, M.J., 1986. Distribution of maximum burial temperatures across northern Appalachian basin and its implication for Carboniferous sedimentation patterns. Geology, 14: 384-387. Kent, D.V., 1979. Paleomagnetism of the Devonian Onondaga Limestone revisited. J. Geophys. Res., 84: 35763588. Kohn, B.P., Wagner, M.E., Lutz, T.M. and Organist, G., 1988a. Regional thermal overprinting during Mesozoic rifting recorded by sphene and zircon fission-track dates, central Appalachian piedmont, USA. Abstr. 6th Int. Conf. Fission-Track Dating, C2-11. Kohn, B.P., Wagner, M.E., Lutz, T.M., Organist, G. and Omar, G., 1988b. Mesozoic rift-related and post-rift uplift of the central Appalachian piedmont: constraints from fission-track analysis. Geol. Soc. Am., Abstr. Program., 20: A387. Lakatos, S. and Miller, D.S., 1983. Fission-track analysis of apatite and zircon defines a burial depth of 4 to 7 km for lowermost Upper Devonian, Catskill Mountains, New York. Geology, 11: 103-104. Levine, J.R., 1986. Deep burial of coal-bearing strata, Anthracite region, Pennsylvania: sedimentation or tectonics: Geology, 14: 577-580. Miller, D.S. and Duddy, I.R., 1989. Early Cretaceous uplift and erosion of the Northern Appalachian Basin, New York, determined from apatite fission-track analysis. Earth Planet. Sci. Lett., 93: 35-49. Miller, D.S., Duddy, I.R., Green, P.F., Hurford, A.J. and Naeser, C.W., 1985. Results of interlaboratory comparison of fission-track age standards: Fission-track Workshop, 1984. Nucl. Tracks, 10: 383-391. Miller, J.D. and Kent, D.V., 1988. Regional trends in the timing of Alleghenian remagnetization in the Appalachians. Geology, 16: 588-591. Naeser, C.W., 1981. The fading of fission-tracks in the geo-
APATITEFISSION-TRACKTHERMOCHRONOLOGYOFTHE PENNSYLVANIAAPPALACHIANBASIN logic environment-data from deep drill holes (unrefereed abstract). Nucl. Tracks, 5: 248-260. Naeser, C.W. and Faul, H., 1969. Fission-track annealing in apatite and sphene. J. Geophys. Res., 74: 705-710. Oliver, J., 1986. Fluids expelled tectonically from orogenic belts: Their role in hydrocarbon migration and other geologic phenomena. Geology, 14: 99-102. Orkan, N.I. and Voight, B., 1983. Fracture geometry and vein mineral paleothermometry-barometry from fluid inclusions, Valley and Ridge Province, Pennsylvania. Geol. Soc. Am., Abstr. Program., 15: 656. Palmer, A.R., 1984. The Decade of North American Geology 1983 Time Scale. Geol. Soc. Am., Map Chart Set., MC-50. Parrish, J.B. and Lavin, P.M., 1982. Tectonic model for kimberlite emplacement in the Appalachian Plateau of Pennsylvania. Geology, 10: 344-347. Paxton, S.T., 1983. Relationships between Pennsylvanianage lithicsandstone and mudrock diagenesis and coal rank in the centralAppalachians. Ph.D. Thesis, Pennsylvania State University,University Park, Pa.,503 pp. Roden, M.K., 1989. Apatite fission-trackthermochronology of the central and southern Appalachian Basin.
51
Ph.D. Thesis, Rensselaer Polytech. Inst., Troy, N.Y., 214 pp. Scotese, C.R., Van der Voo, R. and McCabe, C., 1982. Paleomagnetism of the Upper Silurian and Lower DevonJan carbonates of New York State: Evidence for secondary magnetizations residing in magnetite. Phys. Earth Planet. Interiors, 30: 385-395. Slingerland, R. and Furlong, K.P., 1989. Geodynamic and geomorphic evolution of the Permo-Triassic Appalachian Mountains. In: T.W. Gardner and W.D. Sevon (Editors), Appalachian Geomorphology. Geomorphology, 2:23-37 (this volume). Van der Voo, R., 1979. Age of Alleghenian folding in the central Appalachians. Geology, 7: 297-298. Wagner, G.A., 1968. Fission-track dating of apatites. Earth Planet. Sci. Lett., 4: 411-415. Way, J.H., Smith, R.C., II and Roden, M., 1986. Detailed correlations across 175 miles of Valley and Ridge of Pennsylvania using 7 ash beds in the Tioga zone. In: W.D. Sevon (Editor), Guidebook, 51st Annual Field Conference of Pennsylvania Geologists. Pa. Topogr. Geol. Surv., Huntington, pp. 55-72.
39
Apatite Fission-Track Thermochronology of the Pennsylvania Appalachian Basin MARY K. R O D E N a n d D O N A L D S. M I L L E R Department of Geology, Rensselaer Polytechnic Institute, Troy, N Y 12180-3590 (U.S.A.) (ReceivedDecember 12, 1988, accepted after revision May 1, 1989)
Abstract Roden, M.K. and Miller, D.S., 1989. Apatite fission-track thermochronologyof the Pennsylvania Appalachian Basin. In: T.W. Gardner and W.D. Sevon (Editors), Appalachian Geomorphology.Geomorphology,2:39-51. Thirty-four apatite fission-track apparent ages and twenty-four track length distributions for ash bed samples from the Valleyand Ridge Province and Upper Devonian to Upper Pennsylvanian sedimentary samples from the Allegheny Front and AlleghenyPlateau of Pennsylvania suggest that these regions represent different thermal (uplift) regimes as well as different structural provinces. The Valleyand Ridge Province Tioga and Kalkberg ash bed samples yield apatite fission-track apparent ages and track length distributions that indicate early post-Alleghanian (285-270 Ma) cooling and unroofing that began at ~ 250 Ma. Assuming a geothermal gradient of 25 °C km-1, a burial depth of at least 3.4 km can be estimated for all the Pennsylvania samples. At the Alleghenystructural front and on the western AlleghenyPlateau, the apatite fission-track apparent ages ( < 150 Ma) and track length measurements indicate a Late Jurassic-Early Cretaceous thermal event for these samples possibly resulting from a higher geothermal gradient coinciding with kimberlite intrusion at this time along the Greene-Potter Fault Zone. In northeast Pennsylvania on the AlleghenyPlateau, the Upper Paleozoicsedimentary samples yieldapatite fissiontrack apparent ages < 180 Ma. Narrow track length distributions with long mean lengths (13-14 #m) and small standard deviations (1.3/zm) suggest rapid cooling from temperatures > 110°C during the Middle Jurassic-Early Cretaceous for this part of Pennsylvania. This is consistent with the suggesteduplift history of the Catskill Mountain region in adjacent New York State.
Introduction E s t i m a t i o n of paleoburial depths in the Appalachian Basin have been based on c o n o d o n t alteration indices (Harris, 1979), vitrinite reflectance anisotropy (Levine, 1986), fluid inclusions (Orkan and Voight, 1983), apatite fission-track analysis ( L a ka t os and Miller, 1983; Miller an d Duddy, 1989), s e di m ent ar y compaction (Paxton, 1983), a nd geodynamic lithospheric flexure modelling ( B e a u m o n t et al., 1987). T h e results have indicated a general eastward-thickening (i.e., burial t e m p e r a t u r e
0169-555X/89/$03.50
increasing eastward) depositional basin. J o h n s s o n (1986), in an apatite fission-track and clay mineral diagenesis study of the Middle Devonian Tioga Ash Bed in New York State, found evidence of a t h e r m a l anom al y west of Syracuse t h a t could not be explained solely by burial depth. T h e existence of this t h e r m a l anom al y suggested re-evaluation of the concept of a simple eastward-thickening depositional basin. T hi s paper presents a t h e r m a l history for the central Appalachian Basin of Pennsyl vani a based on apatite fission-track t h e r m o c h r o n o l -
© 1989 Elsevier Science Publishers B.V.
40
MARYK. RODENAND DONALDS. MILLER
ogy of samples of primarily the Middle Devonian Tioga Ash Bed, and Upper Devonian Catskill Formation, as well as a few samples of other Upper Paleozoic sedimentary formations. Samples were taken from the Allegheny Plateau, Allegheny Front, and the Valley and Ridge Province of central and eastern Pennsylvania (Fig. 1 ). This sampling procedure was designed to test the concept of increasing temperatures in the eastern Appalachian Basin due to increasing burial depth and to detect any thermal anomalies that exist across the basin. The fission-track method is based on the formation of damage-zones in uranium-bearing minerals resulting from the spontaneous fission of 23sU over geologic time. Apatite contains appropriate levels of uranium for fission-track analysis. A measured fission-track age is a result of both the time interval of track retention and the amount of annealing that has occurred during that time (Green, 1988). The fissiontrack age will be completely reset, if sufficient heat is applied to totally erase all existing fission-tracks in the apatite. Below a certain critical temperature, known
as the closure temperature (Dodson, 1973, 1979), fission-tracks will begin to accumulate. In isotope geochronology, closure temperature represents the time at which the radiogenic daughter product no longer diffuses out of the system but begins to be retained. In the case of fission-tracks in minerals, this is equivalent to the retention of the damaged regions referred to as tracks. For apatite, the closure temperature is estimated to be 100_+20°C (Wagner, 1968; Naeser and Faul, 1969; Gleadow and Duddy, 1981; Naeser, 1981). Apatite fissiontrack apparent ages cited in this study (Table 1; Fig. 1 ) reflect the time those apatites cooled through the closure temperature of 100°C not their depositional age. Fission-track length measurements on confined, spontaneous tracks in apatite combined with the apatite fission-track apparent ages have been used in thermal history interpretations (Gleadow et al., 1983, 1986a, b). Track length distributions from rapidly cooled volcanic apatites which have not been exposed to temperatures > 50 ° C subsequent to initial cooling have mean track length values between
Falls Creek Wyalusing
152
155
"~ ..
A L L E G H E N Y P L A T EAU
• • F~Sl"" ".Avoc a •• " 14 ~ Pittstowd 127 ~ . • U . • ,• / Dutx~s Lock Haven &. - - - - 121 , ./2" 141 L.^ 97. " " BartSdsvilleY • '-°.~135.-~ V A L L E Y & RIDGE ,, ,' 1 8 4 JStroudsburA L L E G H E N Y FRONT ~.&' Selinsgrove "'" ~1177 = ~ ,'" 218 144-~ urazierville"~," • Kitattiny Mt.) 233 Wardville ,," ~Altoona ,'e Old Port 1 8 7 Swatara Gap..-" /~ JohP.stown4 4 " Mapleton 2 4 5 Mi~,..^. 1 5 2 .--> ..;;-:': -~, Powrys
7&8 ,~'~46 2 2 4 = 2 2 5 2 ~ 5 GR I~AT V A L L E Y L ~ Balc~,'HiU ~ Newton-Hamilton -" . .... ~_-~......~ .~ . .,,' Entr/~iklen892~rbisoni.&..~;,.TR[.ASSiC G E T T Y S B U R G ' ~ A S I N Dickey's Mt,/
203,11 -.
233
=
/'
/
,:
~
~,
." ' A i
'
'" /
PIEDMONT -
J
-
BLUE RIDGE 0
50
100 km
Fig. 1. Sample location names for Tioga ( • ), Kalkberg (II), and Upper Paleozoic sedimentary samples ( • ), apatite fissiontrack apparent ages (Ma), and major tectonic provinces in Pennsylvania.
APATITEFISSION-TRACKTHERMOCHRONOLOGYOFTHE PENNSYLVANIAAPPALACHIANBASIN
41
TABLE1 Apatite fission-track apparent ages, mean track lengths, C1/C1 + F ratios, and U concentrations for the Pennsylvania Appalachian samples Sample number (elev.)
Formation
Sample location
Apparent apatite fission-track age in Ma (no. grains)
Mean track length in/~m (no. tracks)
C1/F+C1
U (ppm)
152_ 8 (20) 245 ___13 (15) 225 _+13 (20) 205 _+12 (19) 233 _+15 (19) 224 _+16 (16) 233 + 13 (19) 218 _+14 (20) 187 + 12 (15) 215 _+13 (12) 177 -+ 10 (18) 203 -+22 (15) 246 _+26* (18) 189 +__22* (19)
13.23_+ 2.0 (98) 14.05 _+1.7 (101) 13.17 ___1.9 (98) 13.48 +__1.6 (101) 13.47 +_1.4 (103) 13.52 -+ 1.6 (100) 13.54 _+1.8 (100) 13.74 + 1.5 (100) 13.78 _+1.5 (101) 13.62 _+1.6 (84) 13.22 _+1.6 (60) 13.21 _+1.6 (130) 13.34 _+1.8 (114) 12.46 _+1.7 (26)
0.19
34
0.19
33
14.04_+ 1.2 (99)
Valley and Ridge Province samples 84-92 (171 m) 8500-1 (135 m) 8500-2 (208 m) 8500-3 (104 m) 8500-4 (190 m) 8500-5 (239 m) 8502-2 (294 m) 8502-3 (141 m) 8602-3 (184 m) 8602-4 (214 m) 8602-5 (141 m) 8602-9 (294 m) 8602-10 (306 m) 8602-52 (245 m)
Tioga
Swatara Gap
Tioga
Old Port
Tioga
Newton-Hamilton
Tioga
Midway
Tioga
Dickey's Mt.
Tioga
Mapleton
Tioga
Grazierville
Tioga
Selingsgrove
Tioga
Wardville
Tioga
Orbisonia
Tioga
Stroudsburg
Kalkberg
Hyndman
Kalkberg
Bald Hill
Catskill
Entriken
Northeastern Allegheny Plateau samples 84-79 (404 m) 84-80 (263 m) 84-82 (367 m) 84-83 (251 m) 84-84 (184 m) 84-87 (208 m) 84-89 (588 m)
Shawangunk
Kitattiny Mt.
Catskill
Bartonsville
Catskill
Avoca
Pottsville
Pittstown
Catskill
Falls
Pocono
Wyalusing
144_+ 8 (21) 184 _+17 (14) 127 _+10 (12) 121_+ 7 (20) 141 _+11 (10) 152 _+17
Pottsville
Falls Creek
155 _+12
13.25 _+1.4 (100) 13.63_+1.1 (94) 13.03 ___1.5 (75) 13.60 ___1.2 (103) 13.60 _+1.2 (100)
28 24 28 29 0.16
33 39 44 37
0.19
47
0.21
10
0.20
18
0.06
27
42
MARYK. RODENANDDONALDS. MILLER
TABLE 1 (continued) Sample number (elev.)
Formation
Sample location
Apparent apatite fission-track age in Ma (no. grains)
Mean track length in/~m (no. tracks)
CI/F+CI
U (ppm)
12.25 ± 1.7 (103) 12.32 ± 1.7 (23) 12.38 ± 1.7 (73)
0.05
21
0.01
26
0.02
18
0.01
13
0.02
31
0.003
25
Western Allegheny Front and Allegheny Plateau samples 8702-2 (203 m) 8702-5 (456 m) 8702-7 (612 m) 8702-8 (685 m) 8702-11 (447 m) 8702-16 (220 m)
Catskill
Lockhaven
Catskill
1-80
Glenshaw
Dubois
Burgoon
Altoona
Catskill
Johnstown
Catskill
Powrys
97 ± 10 (10) 135 ± 20* (9) 141 _+12" (20) 44 _+ 7 (10) 78 +_ 8 (15) 140 ± 11 * (20)
12.81 _+1.5 (57) 12.84 ± 1.7 (100)
*The grain ages for these samples exceeded the limits defined by a chi-square test, thus mean ages are given rather than pooled ages.
14 and 15 ]~m and standard deviations that range from 0.8-1.3/lm (Gleadow et al., 1983, 1986a). Confined track length distributions for apatites from plutonic rocks have mean track lengths in the range of 12-14/~m and standard deviations between 1.0 and 2.0/tm (Gleadow et al., 1986a). In studies of apatites from borehole samples of the Otway Formation in the Otway Basin (Gleadow and Duddy, 1981; Gleadow et al., 1983), the mean confined track length shows a progressive and continuous reduction with increasing down-hole temperature from 15/~m for surface and shallow samples to zero at temperatures of 120°C and greater. Gleadow et al. ( 1986b ) show that small degrees of natural annealing produce narrow track length distributions which become progressively shorter and broader as annealing proceeds. These empirically-determined and naturally-observed characteristics of confined track length distributions in apatite described in the previous two paragraphs will be compared to those track length distributions measured in this study of Pennsylvania rocks. Composition (F and C1 concentration) has been shown to affect the individual fission-track
grain ages of apatites (Green et al., 1986). In their study of apatites from the Otway Group sandstone, Green et al. (1986) found the C1rich grains which had been subjected to a thermal history identical to that of the F-rich grains, had fission-track ages closer to the depositional age of the formation. F and C1 concentrations have been determined for representative samples from this study to check the range of C1 variation. Methods
Apatites and zircons for fission-track analysis were isolated after disaggregation from I to 2 kg samples by standard heavy liquid and magnetic separation techniques. Sample preparation procedures for age and trace length measurements were those of Gleadow (1984). Suitable apatite were recovered from 34 out of 44 samples. Twenty-four samples had statistically significant confined track length measurements. Ten-twenty mg aliquots of apatite were mounted in epoxy on glass slides and polished to expose internal grain surfaces. Spontaneous fission-tracks in the apatites were revealed by etching for 20 s in 5M HNO3 at 21 ° C.
APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE PENNSYLVANIA APPALACHIAN BASIN
The external detector method used in this study requires the mount to be covered by a thin sheet of low uranium muscovite secured by tape. The muscovite serves as a detector for the neutron-induced 235U fission fragments. Samples were irradiated with thermal neutrons at either the U.S. Geological Survey TRIGA reactor in Denver or the Oregon State University TRIGA reactor in Corvallis with two pieces of either NBS standard glass SRM 612 or the Coming Glass standard CN1 included at the ends of the irradiation package. A thermal neutron fluence on the order of 8 × 1015 n cm -2 was obtained for apatite samples. Mica detectors were etched in 40% HF for 15 min. Ages were calculated using a weighted mean zeta calibration factor (Fleischer et al., 1975; Hurford and Green, 1983) for standard apatites from the Fish Canyon Tuff, Durango (Mexico), and the Mount Dromedary quartz monzonite (Miller et al., 1985). Track densities (tracks cm-2) of both spontaneous and induced tracks were measured on a Leitz Ortholux microscope at 1600 X (160 × dry objective, 10× oculars). A chi-square statistic was calculated to discriminate between grains that belong to a single age population (p > 0.05) and those that represent a mixed age population (p < 0.05; indicated by an asterisk in Table 1 ). Length measurements on confined horizontal tracks were made at 1563 × (100 X dry objective, 1.25 × microscope tube, 12.5 × oculars) using a Leitz Ortholux microscope with a drawing tube and a Houston Instruments 1011 digitizing pad interfaced with an IBM PC XT computer. Results
Apatite fission-track apparent ages The Tioga Ash Bed and Kalkberg Formation ash bed samples from west-central Pennsylvania display a range of apatite fission-track apparent ages of 215 _+13-246_+ 26 Ma (Table 1; Fig. 1 ). Near the Southern Anthracite Field in
43
east-central Pennsylvania, the Tioga Ash Bed samples yield a younger range of apatite fission-track apparent ages of 152+8-218_+14 Ma. These ages are consistent with the 177 _+10 Ma apatite fission-track apparent age from the easternmost sample at Stroudsburg. A concordant U - P b date of 390.0 _+0.5 Ma (Roden, 1989) for a monazite from bed B (Way et al., 1986) of a Pennsylvania Tioga sample tightly constrains the stratigraphic age for the formation. All apatite fission-track apparent ages determined in this study are significantly younger than this depositional age. The sedimentary samples of Upper Devonian Catskill Formation and other Upper Paleozoic sedimentary formations from the Allegheny Front and Allegheny Plateau yield a range of apatite fission-track apparent ages, 44 _+7-141 _+12 Ma, significantly younger than the Tioga and Kalkberg samples located to the east (Table 1; Fig. 1). Sedimentary samples from the Allegheny Plateau of northeastern Pennsylvania show a range of apatite fissiontrack apparent ages of 121_+7-184_+17 Ma (Table 1; Fig. 1).
Track length distributions The Tioga Ash Bed and Kalkberg Formation ash bed samples from the Valley and Ridge Province of Pennsylvania have measured track lengths that range from 13.2 to 14.0/~m with the mean standard deviation of the track length distributions ranging from 1.4 to 2.0/tm (Table 1). The mean track length is 13.5_+0.3 /tm (probable error of the mean; Table 1 ). The frequency distributions are characterized by negative skewness and similar histogram shape (Fig. 2a). The Upper Paleozoic sedimentary samples from the Allegheny Plateau of northeastern Pennsylvania (northeast plateau) yield a mean length of 13.5 + 0.3/an consistent with the mean track lengths for the ash bed samples but have a much smaller standard deviation of 1.3/tin (Table 1; Fig. 2b).
44
MARY K. RODEN AND DONALD S. MILLER
(e)
40 '1-
Tioga-Old Port, Pa. No. Tracks = 101
0
30
Mean = 14.05
Std. Dev. = 1.68
~5
20
0
10"
D"
Compositional variation
0 dl
Track Length (microns)
(b) Pocono-Wyalusing, Pa. 40 tO
No. Tracks = 103
3O
Mean = 13.60 Std. Dev. = 1.21
2O O"
10-
0
These differences, combined with the distinctly younger apatite fission-track apparent ages for the Allegheny Front and Allegheny Plateau samples (Table 1) suggest that they have experienced a different thermal history from that of the Valley and Ridge Province ash bed samples and the sedimentary samples from northeastern Pennsylvania.
t o t n ~ t ~ t ~ ~ t¢~ t ~ t o t ~ LgJ ~ t ~ ~ ~tP a'~ w~ a'~ t n u'~
@
The Tioga Ash Bed and Kalkberg Formation apatites are monocompositional with a range of CI/CI+F composition values from 0.18-0.21 (Table 1 ). The Upper Devonian and Upper Paleozoic samples from the western Allegheny Plateau and Allegheny Front are fluorapatites with a limited range of Cl/C1 + F ratios (0.0030.06; Table 1). Because of the limited range of C1/CI+F content of the apatites studied, the fission-track apparent ages and track length reductions measured will be the result of thermal effects without the bias of compositional variation.
Track Length (microns)
(c) Catskill-Lock Haven, Pa. 40 ]
30
I
No. Tracks = 103
Valley and Ridge Province
Mean = 12.25 Std. Dev. = 1.66
20" c
10LL
0
Discussion
~ m ~ m m m ~ m ~ m ~ m m m ~ m
Track Length (microns)
Fig. 2a. Track length distribution for (a) Tioga Ash Bed sample for Old Port, Pa.; ( b ) Pocono Formation sample from Wyalusing, Pa.; and (c) Catskill Formation sample from Lock Haven, Pa.
In contrast, the Upper Devonian and Upper Paleozoic samples from the Allegheny Front and western Allegheny Plateau yield a shorter mean track length of 12.5 _+0.3/Jm and a large standard deviation of 1.6 /xm (Table 1; Fig. 2c).
In Fig. 3, the correlation between coal rank, estimated burial depth (Beaumont et al., 1987) and apatite fission-track apparent age is shown. Apatite fission-track apparent ages for the Tioga samples decrease slightly from west to east as maximum estimated burial depth increases from 5 km at Grazierville (233 _+13 Ma) to 9 km at Stroudsburg (177+10 Ma). The Tioga samples with the youngest apatite fission-track apparent ages are located in the vicinity of the Southern Anthracite Field where both Beaumont et al. (1987) and Levine ( 1986 ) estimate burial depths to be as great as 9 km. Using a geothermal gradient of 25 ° C k m {Beaumont et al., 1987) and a surface temperature of 15°C, a burial depth of at least 3.4 km can be estimated for all the Pennsylvania samples. This is a minimum estimate of burial depth using the apatite fission-track closure temper-
APATITEFISSION-TRACKTHERMOCHRONOLOGYOF THE PENNSYLVANIAAPPALACHIANBASIN
Bituminous Coal Fields
Anthracite Coal F~elds
HighVolath Medium Volatle Low Volatile 0 50 IL
45
,
~ ~
Anthradte Semi-Anthracite
100 km ,
Fig. 3. Apatite fission-track.apparent ages (Ma) for Tioga ( • ) , Kalkberg ( • ) , and Upper Paleozoic sedimentary samples ( • ) from Pennsylvania, estimated maximum burial depth contours in parentheses (kin) (Beaumont et al., 1987), and coal rank.
ature of 100 ° +_20°C. It is consistent with the models of Beaumont et al. (1987) and Levine (1986), however, their maximum burial depth estimates cannot be confirmed because the temperature at 5-9 km depth is >> 100°C (fission-track retention in apatite). Both paleomagnetic ages of 285-270 Ma (Late Pennsylvanian-Early Permian, Van der Voo, 1979; Kent, 1979; Scotese et al., 1982) and vitrinite anisotropy data (Levine, 1986) limit the overburden emplacement and anthracite formation to a 10-15 Ma period. The apatite fission-track apparent ages for the ash bed samples from the Valley and Ridge Province (187+_11-246+_ 26 Ma), with the exception of the Swatara Gap and Stroudsburg samples (Table 1 ), suggest that uplift and erosion in this region began immediately after Alleghenian folding (285-270 Ma) which is consistent with the estimated short time interval (15 Ma) for maximum burial and deformation. The apatite
fission-track apparent ages are also consistent with the calculated erosion period for the ancestral Appalachians (Slingerland and Furlong, this volume). The apatite fission-track apparent ages of ~ 220 Ma indicate an overburden of approximately 3-4 km (depending on estimated geothermal gradient) existed at that time. Slingerland and Furlong (this volume) suggest that the ancestral Appalachians would have been eroded to their present level by 230 Ma which may be consistent with the fissiontrack data within the errors of the model assumptions. The Tioga samples from Swatara Gap and Stroudsburg with the youngest apatite fissiontrack apparent ages (152 + 8 and 177 + 10 Ma, respectively) are located on the southeastern edge of the Valley and Ridge Province near the Triassic Gettysburg Basin (Fig. 1). Their younger apatite fission-track apparent ages may reflect the effects of higher heat flow, higher pa-
46
leogeothermal gradient, in this region during the Mesozoic as suggested by Kohn et al. (1988a, b). Reset fission-track ages for sphene (mean age= 197 Ma) and zircon {mean age= 182 Ma) for samples of Grenville and Taconic crystalline rocks and the Mesozoic Newark Series from the Piedmont and southeastern Pennsylvania have been reported by Kohn et al. (1988a, b). The apatite fission-track apparent ages for their samples have a mean age of 146 Ma and mean track lengths of 13-14/lm. They attribute these fission-track ages to rapid cooling following a high thermal regime (50°C km -1) during the Late Triassic-Early Jurassic extension along the Atlantic margin. From the zircon and sphene fission-track apparent ages, Kohn et al. (1988a, b) estimate 610 km of uplift in the central Appalachian Piedmont since early Jurassic time. Their apatite fission-track apparent ages and track length distributions indicate a discrete phase of Late Jurassic-Early Cretaceous uplift. The apatite fission-track apparent ages for the Swatara Gap and Stroudsburg samples are consistent with a Middle to Late Jurassic uplift in southeastern Pennsylvania. The measured track length distributions for the Valley and Ridge Province samples have long mean lengths (13.2-14.1 ttm) and large standard deviations (1.3-2.0 ttm; Table 1 ). This combined with a frequency distribution shape that is skewed toward short track lengths (Fig. 2a) suggests a slowly decreasing cooling path from temperatures of ~ 120°C to present surface temperature (15 ° C ).
Western Allegheny Front and Allegheny Plateau Figure 3 shows the apatite fission-track apparent ages for the sedimentary samples from the northwestern to northeastern Allegheny Plateau and the Allegheny Front of Pennsylvania and the predicted amounts of post-A1leghenian erosion (Beaumont et al., 1987).
MARY K. RODENAND DONALDS. MILLER
Maximum estimated burial depths vary from 3 km west of the Allegheny Front at Dubois to 9 km in the east near the Northern Anthracite Field. The apatite fission-track apparent ages for all the Allegheny Front and Allegheny Plateau samples are significantly younger than those from the Valley and Ridge Province samples (Fig. 1; Table 1). This is inconsistent with a model of uniform uplift across an eastward deepening depositional basin because the samples in the western part of the basin (western Allegheny Plateau) should have apatite fission-track apparent ages that are the same as, or older than, those of the northeastern Plateau and Valley and Ridge Province samples ( > 246 + 26 Ma). Instead, all the western Allegheny Plateau samples yield apatite fissiontrack apparent ages < 150 Ma. The mean track lengths (12.5+0.3/lm) for the western samples are also shorter than those of both the northeastern Plateau and the Valley and Ridge Province samples (Table 1). Mean track lengths in the 12 #m range with broad frequency distributions suggest slow-cooling with a relatively long time spent in the "track annealing zone" (70 ° -90 ° C ). A mechanism besides burial depth is needed to create a thermal perturbation in the western Allegheny Front and Allegheny Plateau region. There is recent evidence supporting the model of Oliver (1986) for orogenic dewatering due to thrust sheet emplacement which causes topographic highs and forces connate brines toward the craton. The fluid flow resulting from dewatering of the Appalachian Orogen has been suggested as a cause of a Kiaman remagnetization event in the Permo-Carboniferous (310250 Ma) (Miller and Kent, 1988; Jackson et al., 1988), to form authigenic K-feldspars (322-278 Ma, 4°Ar/39Ar ages) (Hearn and Sutter, 1985; Hearn et al., 1987 ), and to act as an illitization agent on mixed-layer illite-smectite in Middle Ordovician K-bentonites with K / A r ages of illitization between 272-303 Ma (Elliott and Aronson, 1987; Altaner, 1985).
47
APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE PENNSYLVANIA APPALACHIAN BASIN
Fluids with temperatures in the range of 100°-200°C would have reset the apatite fission-track ages in sediments that they contacted. This means that the fission-track t T history from depositional age of the sediments (374-300 Ma; Palmer, 1984) to fluid infiltration age ( ~ 250-300 Ma) would be erased. The apatite fission-track apparent ages for the western Allegheny Front and Allegheny Plateau samples should be < 245 Ma representing the time when the sediments had cooled to < 100 °C to allow fission-track accumulation. However, the Allegheny Front and Allegheny Plateau samples have apatite fission-track apparent ages that range from 44 + 7 to 141 + 12 Ma (Fig. 1; Table 1 ). This indicates that there has been post-Permian thermal activity in this region a n d / o r the sediments remained buried to depths where the temperature was 70 °-90 ° C (track annealing zone) until the Late JurassicEarly Cretaceous ( < 150 Ma). There is evidence for both these occurrences.
The Allegheny Front and Allegheny Plateau samples from western Pennsylvania are located along an extension of the Rome Trough and near the cross-structural lineaments presented by Parrish and Lavin (1982) (Fig. 4). This corresponds to the keel-line trend of Dennison (1978, 1983). Parrish and Lavin (1982) suggested that the coincidence of the Pennsylvania and New York kimberlite intrusions and the down-to-the-east basement faults and cross-structural lineaments of the Rome Trough indicate reactivation of these faults in response to the Jurassic extensional opening of the Atlantic Ocean. Basu et al. (1984) have presented K-At age determinations for the Syracuse and Ithaca, New York kimberlites which yield an age range of 113 + 11-176 + 17 Ma. This range of K-Ar ages coincides with the range of apatite fission-track apparent ages ( 4 4 + 7 - 1 4 1 + 1 2 Ma) determined for the Allegheny Front and Allegheny Plateau samples.
//
Greene--PotterFaultZone /
~ , /
/ Allegheny Plateau . ~ ~
1
4
1
,
/
"-... /
\
0 w
r' "
,,, " ""',
"'
,*,,.
Dixonvile"0.
Kimberf~es
~-s-""
• " " "t4-O- - - -
"x<
Allegheny Front
Paleozoic S e d i m e n t s F ~ T r a c k Ages(Ma.)
•
50
m
100
i
•
km
Fig. 4. Greene-Potter fault zone, cross-structural lineaments: T - M U = Tyrone-Mt. Union, P - W-- Pittsburgh-Washington, H-G = Home-Gallitzen, B - B = BlairsviUe-Broad Top, R = Roen discontinuity, L - A = Lawrenceville-Attica (Parrish and Lavin, 1982), apatite fission-track apparent ages (Ma), and kimberlite locations.
48
MARY K. RODEN AND DONALD S. MILLER
Glenshaw-Dubois, Pa. o~ ""
No. Tracks = 73
O
'.= "~ J~
~5 >,
Mean = 12.38 30
Std. Dev. = 1.68
20'
u c
g
10
tL "0
~
Northeastern Allegheny Plateau
40
o. - . - - - - ~ - . k ~ d#dd4dd~d~d~d4~dKdd Track
Length
(microns)
Fig. 5. Bimodal track length distribution for a Glenshaw Formation sample from Dubois, Pa.
The intrusion of the kimberlites themselves could not generate enough heat on a regional scale to overprint a post-Permian cooling age (245 Ma) in these sediments. However, the widespread occurrence of these kimberlites along this zone suggests the possibility of an increased thermal gradient associated with regional igneous intrusive activity in the Rome Trough during the Jurassic. The presence of an intersecting fault zone and lineaments provides tectonic pathways for fluid migration. If the fluids were in the temperature range of 70 °90°C, they could cause annealing of fissiontracks in the apatites they contacted. Additional evidence for a post-Permian heating event on the western Allegheny Plateau of Pennsylvania can be seen in the track length distribution for a Glenshaw Formation from Dubois (Fig. 5). It has a bimodal track length distribution with a shorter peak at 10.5 ttm. This type of length distribution is characteristic of apatites which have spent a prolonged period in the 80°C temperature range (Green et al., 1989). It was during the time at 80 °C that the 10.5 pm peak in the bimodal distribution formed. The temperature was not hot enough to completely anneal the tracks formed since the initial post-Permian cooling, but it was sufficiently high to shorten the tracks to the 10/lm range. The higher peak at 13.5 pm formed after the temperature dropped below 80 ° C.
More rapid cooling (uplift) is indicated for the Upper Devonian samples from the northeastern Allegheny Plateau in their narrow length distributions having a mean standard deviation of 1.3 /tm and a long mean length (13.5 + 0.3 #m). Apatite fission-track apparent ages ( 121 + 17-184 _+17 Ma) for these samples are consistent with a Middle Jurassic to Early Cretaceous uplift as suggested by Miller and Duddy (1989) based on apatite fission-track apparent ages for Upper Devonian samples from the Catskill Mountains to the northeast in New York State. Cl/Cl + F ratios It is doubtful that the apatite fission-track age difference between the Allegheny Front and Allegheny Plateau apatites and the Valley and Ridge Province apatites is due solely to compositional differences. All apatites analyzed in this study comprise a very narrow compositional range (C1/CI+F=0.003-0.21) compared to the Otway Group apatites analyzed by Green et al. {1986) (C1/Cl+F=0.01-0.51). Because the Pennsylvania apatites lack a wide range of C1 concentrations, they do not show a significant correlation between increasing C1/ C1 + F ratio and increasing apatite fission-track apparent ages (Roden, 1989) as seen in the Otway Group apatites (Green et al., 1986). Conclusions
Apatite fission-track apparent ages and track length distributions for the Pennsylvania sedimentary and ash bed samples from the Allegheny Front and Allegheny Plateau and Valley and Ridge Provinces suggest that these regions are not only different tectonic provinces but also represent different thermal (uplift) regimes. The Valley and Ridge Province ash bed samples yield apatite fission-track apparent ages and track length distributions that indicate early
APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE PENNSYLVANIA APPALACHIAN BASIN
post-Alleghanian (285-270 Ma) cooling and unroofing beginning at ~ 250 Ma. This is in agreement with the calculated erosion times of Slingerland and Furlong (this volume). Burial depths of at least 3.4 km can be calculated based on an estimated geothermal gradient of 25 °C km -1 for all the Pennsylvania samples. This is consistent with vitrinite anisotropy measurements of Levine (1986) and geodynamic models of Beaumont et al. (1987) and Slingerland and Furlong (this volume) which suggest deep burial (6-10 kin) in the anthracite region followed by rapid unroofing over a 15 Ma period. Interpretation of the thermal history from apatite fission-track thermochronology west of the Allegheny structural front is complicated. The samples from western Pennsylvania along the Allegheny Front and Allegheny Plateau have been reset to give apatite fission-track apparent ages consistent with thermal histories beginning at < 150 Ma. It is possible that these samples have experienced two heating events. One was caused by hot fluid infiltration ( 100 o_ 200 °C) during the time interval ( ~ 300-250 M a ) as a result of orogenic dewatering. These hot fluids would have reset the apatite fissiontrack ages for the sediments from depositional age (374-300 M a ) to _<250 Ma. This event can not be detected by fission-track analyses of apatite due to the following thermal event. A second thermal event at < 150 M a is indicated by the apatite fission-trackapparent ages for these samples. Track length distributions with short mean lengths (12.5 _+0.3 ,,~m),large standard deviations (1.6/~m), and evidence of bimodality indicate that the thermal event caused temperatures to remain in the 80°C temperature range for an extended period during the Late Jurassic-Early Cretaceous. It is suggested that the elevated temperatures m a y have been the resultof a higher geothermal gradient during this time possibly related to kimberliteintrusion along the Greene-Potter Fault Zone and cross-structurallineaments. For the samples from northeastern Pennsyl-
49
vania on the Allegheny Plateau, the narrow track length distributions with small standard deviations (1.3/~m) indicate rapid cooling at < 185 Ma (maximum apatite fission-track apparent age). This is consistent with a Middle Jurassic-Early Cretaceous uplift suggested by Miller and Duddy (1989) for the Catskill Mountain region of adjacent New York State. Rapid uplift in northeastern Pennsylvania as indicated here may correlate with that seen by Kohn et al. (1988a, b) in southeastern Pennsylvania. It may reflect, as Kohn et al. (1988a, b) suggest, post-rifting uplift caused by landward flexural bulging in response to sedimentary loading of the continental margin.
Acknowledgements This research was supported by a Graduate Professional Opportunities Fellowship, 19851987, a Shell Foundation Fellowship, 19871988, a Texaco Research Grant, 1988-1989, a Geological Society of American Research Grant and a Sigma Xi Grant-in-Aid, 1986. We thank John Way of Lock Haven University and Robert C. Smith, II of the Pennsylvania Topographic and Geologic Survey for their help in the sample collection of the Tioga and Kalkberg ash bed samples and I.R. Duddy for measuring apatite fission-track ages and track lengths for the northeastern Pennsylvania samples. We thank W. Sevon, T. Gardner and an anonymous reviewer for their helpful comments and critical reviews.
References Altaner, S.P., 1985. Potassium metasomatism and diffusion in Cretaceous K-bentonite from the disturbed belt, northwestern Montana, and in the Middle Devonian Tioga K-bentonite, eastern U.S.A. Ph.D. Thesis, University of Illinois,Urbana, Ill.,193 pp. Basu, A.R., Rubury, E., Mehnert, H. and Tatsumoto, M., 1984. Sm-Nd, K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution. Contrib. Mineral. PetroL, 86:
35-44. Beaumont, C., Quinlan, G.M. and Hamilton, J., 1987. The
50 Alleghanian orogeny and its relationship to the evolution of the Eastern Interior, North America. In: C. Beaumont and A.J. Tankard (Editors), Sedimentary Basins and Basin-forming Mechanisms. Can. Soc. Petrol. Geol. Mem., 12: 425-445. Dennison, J.M., 1978. Origin of keel-line peridotites in the Appalachian basin. Geol. Soc. Am., Abstr. Program., 10: 387-388. Dennison, J.M., 1983. Reply on "Tectonic model for kimberlite emplacement in the Appalachian Plateau of Pennsylvania". Geology, 11: 252-253. Dodson, M.E., 1973. Closure temperature in cooling geochronological and petrological systems. Contrib. Mineral. Petrol., 40: 259-274, Dodson, M.E., 1979. Theory of cooling ages. In: E. Jaeger and J.C. Hunziker (Editors), Lectures in Isotope Geology. Springer, Berlin, 329 pp. Elliott, W.C. and Aronson, J.L., 1987. AUeghenian episode of K-bentonite illitization in the southern Appalachian basin. Geology, 15: 735-739. Fleischer, R.L., Price, P.B. and Walker, R.M., 1975. Nuclear Tracks in Solids. University of California Press, Berkeley, Calif., 605 pp. Gleadow, A.J.W., Duddy, I.R., Green, P.F. and Lovering, J.F., 1986a. Confined fission-track lengths in apatite-a diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol., 94: 405-415. Gleadow, A.J.W., Duddy, I.R., Green, P.F. and Hegarty, K.A., 1986b. Fission-track lengths in the apatite annealing zone and the interpretation of mixed ages. Earth Planet. Sci. Lett., 78: 245-254. Gleadow, A.J.W., 1984. Fission-track dating methods-II: a manual of principles and techniques. Workshop of Fission Track Analysis: Principles and Applications. James Cook University, Townsville, Qld., 35 pp. Gleadow, A.J.W., Duddy, I.R. and Lovering, J.F., 1983. Fission-track analysis: a new tool for the evolution of thermal histories and hydrocarbon potential. Aust. Petrol. Explor. Assoc. J., 23: 93-102. Gleadow, A.J.W. and Duddy, I.R., 1981. A natural longterm annealing experiment for apatite. Nucl. Tracks, 5: 169-174. Green, P.F., 1988. The relationship between track shortening and fission-track age reduction in apatite: combined influences of inherent instability, annealing anisotropy, and length bias and system calibration. Earth Planet. Sci. Lett., 89: 335-352. Green, P.F., Duddy, I.R., Gleadow, A.J.W., Tingate, P.R. and Laslett, G.M., 1986. Thermal annealing of fissiontracks in apatite: 1. A qualitative description. Chem. Geol. (Isot. Geosci. Sect.), 59: 237-253. Green, P.F., Duddy, I.R., Laslett, G.M., Hegarty, K.A., Gleadow, A.J.W. and Lovering, J.F., 1989. Thermal annealing of fission-tracks in apatite: 4. Quantitative modelling techniques and extension to geological time scales. Chem. Geol. (Isot. Geosci. Sect. ), 79: 155-182.
MARYK. RODENANDDONALDS. MILLER tlarris, A.G., 1979. Conodont color alteration, an organomineral metamorphic index and its application to Appalachian Basin geology. In: P.A. Scholle and P.R. Schluger (Editors), Aspects of Diagenesis. Soc. Econ. Paleontol. Mineral. Spec. Publ., 26: 3-16. Hearn, P.P. and Sutter, J.F., 1985. Authigenic potassium feldspar in Cambrian carbonates: Evidence of Alleghenian brine migration. Science, 228: 1529-1531. Hearn, P.P., Sutter, J.F. and Belkini, H.E., 1987. Evidence for Late-Paleozoic brine migration in Cambrian carbonate rocks of the central and southern Appalachians: Implications for Mississippi Valley type sulfide mineralization. Geochim. Cosmochim. Acta, 51: 1323-1334. Hurford, A.J. and Green, P.F., 1983. The zeta age calibration of fission-track dating. Chem. Geol. (Isotope Geoscience Section), 1: 285-317. Jackson, M., McCabe, C., Ballard, M.M. and Van der Voo, R., 1988. Magnetite authigenesis and diagenetic paleotemperatures across the northern Appalachian basin. Geology, 16: 592-595. Johnsson, M.J., 1986. Distribution of maximum burial temperatures across northern Appalachian basin and its implication for Carboniferous sedimentation patterns. Geology, 14: 384-387. Kent, D.V., 1979. Paleomagnetism of the Devonian Onondaga Limestone revisited. J. Geophys. Res., 84: 35763588. Kohn, B.P., Wagner, M.E., Lutz, T.M. and Organist, G., 1988a. Regional thermal overprinting during Mesozoic rifting recorded by sphene and zircon fission-track dates, central Appalachian piedmont, USA. Abstr. 6th Int. Conf. Fission-Track Dating, C2-11. Kohn, B.P., Wagner, M.E., Lutz, T.M., Organist, G. and Omar, G., 1988b. Mesozoic rift-related and post-rift uplift of the central Appalachian piedmont: constraints from fission-track analysis. Geol. Soc. Am., Abstr. Program., 20: A387. Lakatos, S. and Miller, D.S., 1983. Fission-track analysis of apatite and zircon defines a burial depth of 4 to 7 km for lowermost Upper Devonian, Catskill Mountains, New York. Geology, 11: 103-104. Levine, J.R., 1986. Deep burial of coal-bearing strata, Anthracite region, Pennsylvania: sedimentation or tectonics: Geology, 14: 577-580. Miller, D.S. and Duddy, I.R., 1989. Early Cretaceous uplift and erosion of the Northern Appalachian Basin, New York, determined from apatite fission-track analysis. Earth Planet. Sci. Lett., 93: 35-49. Miller, D.S., Duddy, I.R., Green, P.F., Hurford, A.J. and Naeser, C.W., 1985. Results of interlaboratory comparison of fission-track age standards: Fission-track Workshop, 1984. Nucl. Tracks, 10: 383-391. Miller, J.D. and Kent, D.V., 1988. Regional trends in the timing of Alleghenian remagnetization in the Appalachians. Geology, 16: 588-591. Naeser, C.W., 1981. The fading of fission-tracks in the geo-
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